1. Field of the Invention
The present invention concerns a compressor unit containing a compressor element driven by a motor which is provided with an outlet pipe and an inlet pipe, and a compressed air receiver onto which the outlet pipe is connected, whereby a pneumatically controlled throttle valve is provided in the inlet pipe, whereas the motor has a pneumatically controlled speed regulator and both this speed regulation and the throttle valve are connected to the compressed air receiver via a compressed air pipe and a control device with a control valve in the compressed air pipe.
2. Description of the Related Art
With known compressor units of the above type, the control device contains two valves erected in parallel, namely a pneumatic control valve and an electromechanical load valve. The pipe which is connected to the compressed air receiver via these two valves is connected to the connecting pipe between the speed regulator and the throttle. Onto this connecting pipe are connected branches which are provided with small air holes.
The output of the compressor element depends on the rotational speed of the motor and thus of the speed regulator and the throttle in the inlet pipe.
The rotational speed and the throttle are adjusted by means of the regulating pressure which is built up by the pneumatic control valve on the basis of the pressure in the compressed air receiver.
The nominal pressure, i.e. the operating pressure under full load, is adjusted manually by means of the control valve. If the air receiver pressure is equal to the nominal pressure while load-running, the regulating pressure is zero, the throttle valve is entirely open and the rotational speed of the motor is maximal.
If however, the air receiver pressure is higher, in particular maximal, for example 2 bar above the nominal pressure, the rotational speed is minimal and the throttle valve is entirely closed. The regulating pressure is proportional to the difference between the air receiver pressure and the nominal pressure.
Between no regulating pressure and the maximum regulating pressure, any output can be set between the maximum and zero respectively.
Since the pneumatic control valve only lets air through in one direction, the above-mentioned blow-off holes are necessary. By letting air escape via these blow-off holes, it is possible for the regulating pressure to drop when the air receiver pressure is lowered.
By means of pipe restrictions and volumes to be filled, the regulating pressure dynamically approaches a first-order process. With a lowering and rising load, the variation of the air receiver pressure will be retarded. This results in an overshoot (air receiver pressure too high) when the load diminishes, and in an undershoot (air receiver pressure too low) when the load increases.
The load valve is required in order to be able to start under no-load conditions, with a minimal rotational speed and a closed throttle valve. This load valve, which bridges the regulating valve, is opened when starting, so that the air receiver pressure can act directly on the throttle valve and the speed regulation. The air receiver pressure then amounts to for example 2 bar.
When the compressor element is loaded, the load valve is shut and the regulating pressure is blown off via the blow-off holes, after which the above-described adjustment under load takes place.